The present invention concerns improvements in and relating to roller subs for use downhole in oil or gas wells as part of the tool string or drill string to reduce friction between the string and the wellbore.
Roller subs are used widely throughout the oil industry but especially in wireline toolstrings, which rely on gravity alone to advance the toolstring, and are especially useful down wells that deviate substantially from the vertical.
Conventional roller subs are generally substantially solid circular cylindrical bodies that are milled to provide radial slots at intervals therearound and therealong. These slots each accommodate a respective roller wheel. Two example prior art configurations of multi-roller wheel sub are illustrated in FIGS. 1 and 2 below.
In the FIG. 1 example the cylindrical body 1, formed with the plurality of slots 2, holds each roller wheel 3 in place in its respective slot by means of a grub screw 4 which locks down onto a radius groove machined into the head of a caphead screw 5 that serves as the axle of the respective roller wheel 3.
In the FIG. 2 example, the roller sub has substantially the same configuration but in this case the axle 6 of each roller wheel 3 has an undercut into which a fixing grub screw 7 locks.
A number of practical problems arise from the use of such conventional designs of multiroller wheel sub, perhaps the most important of which is that the axles and the grub screws and other locking fixtures for holding the roller wheels 3 in place are vulnerable to mechanical failure which may lead to jamming of roller wheels or their loss downhole. Loss of mechanical components such as these downhole is, of course, extremely undesirable since they may interfere with operation of the well and necessitate costly interruption of production to attempt to locate and fish them out.
According to the present invention there is provided a roller sub for use downhole in oil or gas wells as part of a toolstring or drill string to reduce friction between the string and wellbore, which roller sub carries at least one roller wheel, wherein the roller sub is a modular assembly of parts which assemble together to trap the at least one roller wheel in place between them.
Preferably the modular assembly comprises a body formed of segments.
Suitably the modular assembly body of the roller sub comprises six segments.
Advantageously each roller wheel has integral (i.e. integrally formed or assembled) pivot pin means.
Preferably the pivot pin means of each roller wheel comprise axle stubs that are domed or substantially hemispherical in shape to co-operatively engage with correspondingly shaped recesses in the body of the roller sub.
Suitably each roller wheel has a circumferential groove whereby the roller wheel has a dumbell-like shape in profile.
Advantageously a channel is provided extending longitudinally through the roller sub to serve as a conduit for fluids or electric line.
Preferably part of the channel is defined by the circumferential grooves of the roller wheels.